Yassen Gregorovich is an assasin and a villain in the Alex Rider series. He was responsible for killing Alex's uncle and Alex looks for him in the entire story for vengeance against Yassen.
Yassen was born in Russia and had a rough childhood. His father died in a biochemical warface accident and his mother died in six months from an illness. He managed to become an successful assasin after he works for the Mafia in Moscow.
He later tracked down SCORPIA, the crime organization, and became a high ranking agent. Then he set off on his own as a freelance assassin. He was with John Rider, Alex's father, (unbeknown to Yassen, as part of John's MI6 cover in SCORPIA) when him and John killed a drug dealer in the Amazon. Then Yassen became even more powerful. He met Ash in an operation in Malta and sliced open Ash's stomach, leaving him in constant pain for the rest of his life. Yassen was the primary reason for Ash's treachery.
As Yassen's bank account was not backed up, he needed money, so he went to Herod Sayle, meeting the businessman in Crete where they exchanged contacts and arranged to kill half of Britain's children. Ian Rider, Alex's uncle, discovered this and knew of Yassen's involvement. He headed back to London after reporting back to MI6 but Yassen intercepted him on a quiet lane and gunned him down. The police later lied about Ian's death. But Alex, his nephew, investigated and was eventually recruited by MI6 into saving the country from the smallpox virus Sayle and Yassen had hidden in the Stormbreaker computers.
Yassen later killed Sayle at the end of the operation as the man had become too embarrasing. He and Alex shared a minor bonding moment where Yassen told him to go back to normality and forget about killing and MI6. He said "killing is for grown ups and you're still a child." So, he said he had no orders to kill Alex, and he expressed that he had no hate for the boy despite killing his uncle which, he explained, was just work for him. Yassen waved goodbye to Alex and flew off on a helicopter.
South of France and Damian Cray
When Yassen was recruited by mad businessman Damian Cray, things got bad. First, the bomb he had put in Edward Pleasure's house didn't kill Edward. What's more, Edward's daughter and her friend Alex saw the explosion and Alex Rider knew it was Yassen having seen him on the beach earlier. He tracked Yassen down and Yassen tried to kill Alex by leaving him in a bull ring but Alex escaped.
When Alex got into Cray's kingdom and discovered Cray was indeed at the heart of it all, Yassen was the only one who looked concerned when Alex was dragged off to be killed. He survived, however, and Yassen told Cray what he knew about the boy. He said that Alex was so useful to MI6 because he was a boy. Then he said that he was with Sabrina Pleasure, Edward's daughter, and Cray decided to kidnap her. Yassen had a bunch of men waiting for Alex when he came out of headquarters but Alex survived them all.
Alex went after Sabrina and Cray revealed his plan to blow up all the drugs headqaurters globally. Yassen would then retire to Moscow. But Alex sowed unrest between Cray and Yassen. He eventually did rebel against Cray, and it cost him his life when Cray gunned him down on Air Force One. Yassen repaid Ian Rider's debt by telling Alex about SCORPIA.